Thermosetting resin composition, prepreg, and fiber-reinforced composite material and production method therefor

ABSTRACT

Provided is a thermosetting resin composition which achieves both high heat resistance and high bending strength as a fiber-reinforced composite material, and also has rapid curability that enables high cycle press forming, thermal stability, and storage stability. The thermosetting resin composition of the present invention is a thermosetting resin composition comprising an epoxy resin, an epoxy resin curing agent, an imidazole compound, and an epoxy resin curing accelerator, in which the epoxy resin curing agent is dicyandiamide or a derivative thereof, and the epoxy resin curing accelerator comprises a urea derivative having two or more dimethylureido groups in a molecule.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 16/861,289, filed Apr. 29, 2020, the disclosure of which isincorporated herein by reference in its entirety. U.S. application Ser.No. 16/861,289 is a continuation application of PCT/JP2018/040504 filedOct. 31, 2018, the disclosure of which is incorporated herein byreference in its entirety. U.S. application Ser. No. 16/861,289 claimspriority to Japanese Application No. 2017-220688, the disclosure ofwhich is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a thermosetting resin composition, aprepreg, and a fiber-reinforced composite material and a productionmethod therefor.

This application is a continuation application of InternationalApplication No. PCT/JP2018/040504, filed on Oct. 31, 2018, which claimsthe benefit of priority of the prior Japanese Patent Application No.2017-220688, filed Nov. 16, 2017, the content of which are incorporatedherein by reference.

BACKGROUND ART

Fiber-reinforced composite materials containing reinforcing fibers and amatrix resin composition are widely used in industrial applications suchas vehicles by reason of excellent mechanical properties and the like,and in recent years, the application range thereof has been furtherwidened.

For example, a fiber-reinforced composite material formed by heating andpressing a prepreg laminate in which a plurality of sheet-like prepregshaving reinforcing fibers impregnated with a matrix resin compositionare laminated is known.

As the matrix resin composition, a thermosetting resin compositioncontaining a phenol resin, a melamine resin, a bismaleimide resin, anunsaturated polyester resin, an epoxy resin, or the like is often usedfrom the viewpoint of excellent impregnation properties and heatresistance.

Particularly, an epoxy resin composition has been widely used because afiber-reinforced composite material having excellent heat resistance andformability and higher mechanical strength can be obtained.

As a forming method frequently used for vehicles, high cycle pressforming is known (Patent Literature 1).

In high cycle press forming, in order to enable mass production ofproducts, the products are cured under a high pressure at about 100° C.to 150° C. for a period of time as short as about several minutes totens of minutes.

A resin composition used for the high cycle press forming is required tohave rapid curability that enables curing within a period of time asshort as about several minutes to tens of minutes at about 100° C. to150° C., thermal stability in which the viscosity properties of theresin do not change while a reinforcing fiber base material isimpregnated with the resin composition, and storage stability duringstorage. However, rapid curability, and thermal stability and storagestability are contradictory properties, and it has been difficult tosatisfy all the properties.

In addition, in high cycle press forming, a formed article has heatresistance higher than a forming temperature in order to preventdeformation at the time of demolding. Furthermore, since the useapplications of the formed article have been widened, a formed articleresistant to high-temperature heat is required.

In general, although the heat resistance of a cured product of an epoxyresin and an imidazole-based compound is high, in a case where an epoxyresin composition containing an imidazole-based compound is used, thebending strength (for example, 90-degree bending strength) of theobtained fiber-reinforced composite material becomes a problem. That is,the cured product of the epoxy resin composition containing theimidazole-based compound has poor adhesion to a reinforcing fiber basematerial. Therefore, in case of the fiber-reinforced composite material,fracture is likely to occur at the interface between the cured productof the epoxy resin composition and the reinforcing fiber base material.

On the other hand, in a case where dicyandiamide or a derivative thereofis used as a curing agent, the adhesion to the reinforcing fiber basematerial is strong, and high strength can be exhibited in a bending test(for example, a 90-degree bending test) of the obtained fiber-reinforcedcomposite material. However, in the case where dicyandiamide or aderivative thereof is used as the curing agent, the heat resistance ofthe cured product is not high.

Patent Literature 2 discloses an example in which an imidazole-basedcompound, and dicyandiamide or a derivative thereof are used as a curingagent.

CITATION LIST Patent Literature

[Patent Literature 1]

-   International publication under the PCT numbered WO 2004/48435    [Patent Literature 2]-   International publication under the PCT numbered WO 2016/199857

SUMMARY OF INVENTION Technical Problem

However, the heat resistance of the thermosetting resin compositionsdisclosed in these literatures is not high. Furthermore, in high cyclepress forming, compatibility between heat resistance and bendingproperties has not been achieved.

The present invention has been made in view of the above problems, andan object thereof is to provide a thermosetting resin composition whichachieves both high heat resistance and high bending strength as afiber-reinforced composite material, and also has rapid curability,thermal stability, and storage stability that enable high cycle pressforming, and a prepreg, and a fiber-reinforced composite material usingthe same.

Solution to Problem

The present invention has the following aspects.

[1] A thermosetting resin composition comprising: an epoxy resin; anepoxy resin curing agent; an imidazole compound; and an epoxy resincuring accelerator, wherein

the epoxy resin curing agent is dicyandiamide or a derivative thereof,and

the epoxy resin curing accelerator comprises a urea derivative havingtwo or more dimethylureido groups in a molecule.

[2] The thermosetting resin composition according to [1], wherein acuring completion time measured by a curelastometer is 6 minutes orshorter at 140° C.

[3] The thermosetting resin composition according to [1] or [2],wherein, when a viscosity after holding at 60° C. for 5 minutes is V0and a viscosity after holding at 60° C. for 5 hours is V1, V1/V0 is 1.0to 4.5.

[4] The thermosetting resin composition according to any one of [1] to[3], wherein the imidazole compound comprises an imidazole compoundhaving a curing start temperature of 100° C. or higher,

[Curing Start Temperature of Imidazole Compound]

A curing start temperature of an imidazole compound is a temperature atan intersection between a tangent at an inflection point and a baselinein a DSC curve which is obtained by measuring a caorific value by adifferential scanning calorimeter (DSC) at a temperature rising rate of10° C./min of a sample resin composition prepared by mixing 10 parts bymass of the imidazole compound with 100 parts by mass of a bisphenol Atype epoxy resin having an epoxy equivalent weight of 180 to 220.

[5] The thermosetting resin composition according to any one of [1] to[4], wherein the imidazole compound comprises one selected from thegroup consisting of an imidazole derivative, an imidazole adduct, aninclusion compound of imidazole, a microencapsulated imidazole, and anaddition product comprising imidazole.

[6] The thermosetting resin composition according to [5], wherein theimidazole compound comprises the addition product comprising imidazole,and the addition product comprising imidazole is an isocyanuric acidaddition product of imidazole or an isocyanuric acid addition product ofan imidazole derivative.

[7] The thermosetting resin composition according to any one of [1] to[6], wherein the urea derivative having two or more dimethylureidogroups in a molecule comprises one or both of2,4-bis(3,3-dimethylureido)toluene and 4,4′-methylenebis(phenyldimethylurea).

[8] The thermosetting resin composition according to any one of [1] to[7], the thermosetting resin composition comprising:

1 to 9 parts by mass of dicyandiamide or a derivative thereof,

1 to 8 parts by mass of the imidazole compound, and

1 to 8 parts by mass of the urea derivative having two or moredimethylureido groups in a molecule with respect to 100 parts by mass ofthe epoxy resin.

[9] The thermosetting resin composition according to any one of [1] to[8], wherein the epoxy resin comprises a polyfunctional epoxy resinhaving three or more epoxy groups in a molecule.

[10] The thermosetting resin composition according to [9], wherein thepolyfunctional epoxy resin having three or more epoxy groups in amolecule comprises one selected from the group consisting of a phenolnovolac type epoxy resin, a trisphenolmethane type epoxy resin, andtetraglycidyl diaminodiphenylmethane.

[11] The thermosetting resin composition according to any one of [1] to[10], wherein the epoxy resin comprises 30 to 70 parts by mass of anepoxy resin having a structure represented by Formula (1) in a molecule,and 20 to 40 parts by mass of a polyfunctional epoxy resin having threeor more epoxy groups in a molecule without having the structurerepresented by Formula (1), in 100 parts by mass of the epoxy resin.

[12] A prepreg comprising: the thermosetting resin composition accordingto any one of [1] to [11]; and a reinforcing fiber base material.

[13] A fiber-reinforced composite material which is a cured product ofthe prepreg according to [12].

[14] A production method of a fiber-reinforced composite materialcomprising: producing a preform having a near net shape of a desiredformed article shape by shaping one sheet or two or more overlappingsheets of the prepreg according to [12]; and performing heating andpressurization forming on the preform into the desired formed articleshape.

[15] The thermosetting resin composition according to any one of [1] to[11], which is used for a vehicle material.

Advantageous Effects of Invention

According to the present invention, a thermosetting resin compositionwhich gives a fiber-reinforced composite material with both high heatresistance and high bending strength and such a fiber-reinforcedcomposite material may be obtained.

Furthermore, the thermosetting resin composition of the presentinvention also has rapid curability that enables high cycle pressforming, and thermal stability and storage stability required for aprepreg.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view illustrating an example of a prepreglaminate.

FIG. 2 is a step diagram illustrating an example of a forming step in aproduction method for a fiber-reinforced composite material.

FIG. 3 is a cross-sectional view illustrating an example of thefiber-reinforced composite material.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed, but the present invention is not limited to only theseembodiments.

<<Thermosetting Resin Composition>>

A thermosetting resin composition of the present invention contains anepoxy resin, an epoxy resin curing agent, an imidazole compound, and anepoxy resin curing accelerator.

Here, the epoxy resin curing agent is dicyandiamide or a derivativethereof.

The epoxy resin curing accelerator contains a urea derivative having twoor more dimethylureido groups in a molecule.

By using dicyandiamide or a derivative thereof as the epoxy resin curingagent, the mechanical properties of a cured product of the thermosettingresin composition are improved.

By using the imidazole compound, the curing rate of the thermosettingresin composition is improved, and the mechanical properties of theobtained cured product are improved.

By using the urea derivative having two or more dimethylureido groups ina molecule as the epoxy resin curing accelerator, the low-temperaturecurability and rapid curability of the thermosetting resin compositionare improved.

The thermosetting resin composition of the present invention can achieveboth rapid curability and storage stability by containing all ofdicyandiamide or a derivative thereof, the imidazole compound, and theurea derivative having two or more dimethylureido groups in a molecule.

[Epoxy Resin]

As the epoxy resin used in the present invention, there is a compoundhaving an epoxy group in a molecule. A compound having two or more epoxygroups in a molecule is preferable because the obtained cured resinproduct has high heat resistance and high mechanical properties.

Examples of the epoxy resin include: a glycidyl ether type epoxy resinobtained from a compound having a hydroxyl group in a molecule andepichlorohydrin; a glycidyl amine type epoxy resin obtained from acompound having an amino group in a molecule and epichlorohydrin; aglycidyl ester type epoxy resin obtained from a compound having acarboxyl group in a molecule and epichlorohydrin; an alicyclic epoxyresin obtained by oxidizing a compound having a double bond in amolecule; an epoxy resin having a heterocyclic structure; and an epoxyresin in which two or more types of groups selected from these are mixedin a molecule.

As an epoxy resin other than these, an epoxy resin having a structurerepresented by Formula (1) in a molecule can also be used.

One of the epoxy resins may be used alone, or two or more thereof may beused in combination.

(Glycidyl Ether Type Epoxy Resin)

Specific examples of the glycidyl ether type epoxy resin include: anaryl glycidyl ether type epoxy resin such as a bisphenol A type epoxyresin, a bisphenol F type epoxy resin, a bisphenol S type epoxy resin, aresorcinol type epoxy resin, a phenol novolac type epoxy resin, atrisphenolmethane type epoxy resin, a naphthalene type epoxy resin, andan anthracene type epoxy resins; a polyethylene glycol type epoxy resin;a polypropylene glycol type epoxy resin; a dicyclopentadiene type epoxyresin; and positional isomers thereof or substituted products with alkylgroups or halogens.

Examples of commercial products of the bisphenol A type epoxy resininclude: EPON 825; jER (registered trademark; the same applieshereinafter) 826, jER 827, and jER 828 (all manufactured by MitsubishiChemical Corporation); EPICLON (registered trademark; the same applieshereinafter) 850 (manufactured by DIC Corporation); EPOTOHTO (registeredtrademark; the same applies hereinafter) YD-128 (manufactured by NIPPONSTEEL Chemical & Material Co., Ltd.); DER-331 and DER-332 (bothmanufactured by The Dow Chemical Company); and Bakelite EPR 154,Bakelite EPR 162, Bakelite EPR 172, Bakelite EPR 173, and Bakelite EPR174 (all manufactured by Bakelite AG).

Examples of commercial products of the bisphenol F type epoxy resininclude: jER 806, jER 807, and jER 1750 (all manufactured by MitsubishiChemical Corporation); EPICLON 830 (manufactured by DIC Corporation);EPOTOHTO YD-170 and EPOTOHTO YD-175 (both manufactured by NIPPON STEELChemical & Material Co., Ltd.); Bakelite EPR169 (manufactured byBakelite AG); and GY 281, GY 282, and GY 285 (all manufactured byHuntsman Advanced Materials).

Examples of commercial products of the bisphenol S type epoxy resininclude EPICLON EXA-1514 (manufactured by DIC Corporation).

Examples of commercial products of the resorcinol type epoxy resininclude DENACOL (registered trademark; the same applies hereinafter)EX-201 (manufactured by Nagase ChemteX Corporation).

Examples of commercial products of the phenol novolac type epoxy resininclude: jER 152 and jER 154 (both manufactured by Mitsubishi ChemicalCorporation); EPICLON N-740 (manufactured by DIC Corporation); and EPN179 and EPN 180 (both manufactured by Huntsman Advanced Materials).

Examples of commercial products of the trisphenolmethane type epoxyresin include: Tactix (registered trademark; the same applieshereinafter) 742 (manufactured by Huntsman Advanced Materials); EPPN(registered trademark; the same applies hereinafter) 501H, EPPN 501HY,EPPN 502H, and EPPN 503H (all manufactured by Nippon Kayaku Co., Ltd.);and jER 1032H60 (manufactured by Mitsubishi Chemical Corporation).

Examples of commercial products of the naphthalene type epoxy resininclude: HP-4032 and HP-4700 (both manufactured by DIC Corporation); andNC-7300 (manufactured by Nippon Kayaku Co., Ltd.).

Examples of commercial products of the dicyclopentadiene type epoxyresins include XD-100 (manufactured by Nippon Kayaku Co., Ltd.) and HP7200 (manufactured by DIC Corporation).

Examples of commercial products of the anthracene type epoxy resininclude YL7172YX-8800 (manufactured by Mitsubishi Chemical Corporation).

(Glycidyl Amine Type Epoxy Resin)

Specific examples of the glycidyl amine type epoxy resin includetetraglycidyl diaminodiphenylmethanes, a glycidyl compound ofaminophenol, a glycidyl compound of aminocresol, glycidylanilines, and aglycidyl compound of xylenediamine.

Examples of commercial products of the tetraglycidyldiaminodiphenylmethanes include: SUMI-EPDXY (registered trademark; thesame applies hereinafter) ELM 434 (manufactured by Sumitomo ChemicalCompany); ARALDITE (registered trademark; the same applies hereinafter)MY 720, ARALDITE MY 721, ARALDITE MY 9512, ARALDITE MY 9612, ARALDITE MY9634, and ARALDITE MY 9663 (manufactured by Huntsman Advanced MaterialsCo., Ltd.); jER 604 (manufactured by Mitsubishi Chemical Corporation);and Bakelite EPR 494, Bakelite EPR 495, Bakelite EPR 496, and BakeliteEPR 497 (all manufactured by Bakelite AG).

Examples of commercial products of the glycidyl compound of aminophenoland the glycidyl compound of aminocresol include: jER 630 (manufacturedby Mitsubishi Chemical Corporation); ARALDITE MY 0500, ARALDITE MY 0510,ARALDITE MY 0600 (all manufactured by Huntsman Advanced Materials); andSUMI-EPDXY ELM 120 and SUMI-EPDXY ELM 100 (all manufactured by SumitomoChemical Company).

Examples of commercial products of the glycidylanilines include: GAN(registered trademark; the same applies hereinafter) and GOT (registeredtrademark; the same applies hereinafter) (manufactured by Nippon KayakuCo., Ltd.); and Bakelite EPR 493 (manufactured by Bakelite AG).

Examples of the glycidyl compound of xylenediamine include TETRAD(registered trademark; the same applies hereinafter)-X (manufactured byMitsubishi Gas Chemical Company, Inc.).

(Glycidyl Ester Type Epoxy Resin)

Specific examples of the glycidyl ester type epoxy resin includephthalic acid diglycidyl ester, hexahydrophthalic acid diglycidyl ester,isophthalic acid diglycidyl ester, dimer acid diglycidyl ester, andisomers of kinds thereof.

Examples of commercial products of the phthalic acid diglycidyl esterinclude: EPOMIK (registered trademark; the same applies hereinafter)R508 (manufactured by Mitsui Chemicals, Inc.) and DENACOL (registeredtrademark; the same applies hereinafter) EX-721 (manufactured by NagaseChemteX Corporation).

Examples of commercial products of the hexahydrophthalic acid diglycidylester include EPOMIK R540 (manufactured by Mitsui Chemicals, Inc.) andAK-601 (manufactured by Nippon Kayaku Co., Ltd.).

Examples of commercial products of the dimer acid diglycidyl esterinclude jER 871 (manufactured by Mitsubishi Chemical Corporation) andEPOTOHTO YD-171 (manufactured by NIPPON STEEL Chemical & Material Co.,Ltd.).

(Alicyclic Epoxy Resin)

Specific examples of the alicyclic epoxy resin include a compound havinga 1,2-epoxycyclohexane ring as a partial structure.

Examples of commercial products of the compound having a1,2-epoxycyclohexane ring as a partial structure include: CELLOXIDE(registered trademark; the same applies hereinafter) 2021P, CELLOXIDE2081, and CELLOXIDE 3000 (all manufactured by Daicel Corporation); andCY 179 (manufactured by Huntsman Advanced Materials).

(Epoxy Resin Having Heterocyclic Structure)

Specific examples of the epoxy resin having a heterocyclic structureinclude a compound having an oxazolidone ring as a partial structure,and a compound having a xanthene skeleton as a partial structure.

Examples of commercial products of the compound having an oxazolidonering as a partial structure include: AER (registered trademark; the sameapplies hereinafter) 4152, AER 4151, LSA 4311, LSA 4313, and LSA 7001(all manufactured by Asahi Kasei E-Materials Co.); and EPICLON TSR-400(manufactured by DIC Corporation).

Examples of commercial products of the compound having a xantheneskeleton as a partial structure include EXA-7335 (manufactured by DICCorporation).

(Epoxy Resin Having Structure Represented by Formula (1))

Examples of the epoxy resin having a structure represented by Formula(1) include a reaction product of an epoxy resin and an amine compoundhaving at least one sulfur atom in a molecule.

Examples of the epoxy resin having the structure represented by Formula(1) include 4,4′-diaminodiphenylsulfone, 3,3′-diaminodiphenylsulfone,bis(4-(4-aminophenoxy)phenyl)sulfone,bis(4-(3-aminophenoxy)phenyl)sulfone, and derivatives thereof.

Among these, from the viewpoint of heat resistance of the cured resin,it is preferable to use diaminodiphenylsulfone, and it is morepreferable to use 4,4′-diaminodiphenylsulfone.

Examples of the epoxy resin preferably used include a bisphenol typeepoxy resin such as a bisphenol A type epoxy resin and a bisphenol Ftype epoxy resin. Among these, a bisphenol A type epoxy resin ispreferable.

As a method for obtaining the epoxy resin having the structurerepresented by Formula (1), there is a method of mixing an epoxy resinwith an amine compound having at least one sulfur atom in a molecule,specifically, an amine compound having the structure represented byFormula (1) in a mass ratio of 100:3 to 100:30, preferably 100:5 to100:20, and heating the mixture at 130° C. to 200° C., preferably 140°C. to 170° C. to cause a reaction.

In a case where this method is used, unreacted epoxy resin and the aminecompound may remain in the reaction product, but there is no particularneed to remove these residues.

By using the epoxy resin having the structure represented by Formula (1)as the epoxy resin in the thermosetting resin composition of the presentinvention, the viscosity of the thermosetting resin composition can beeasily adjusted, which is preferable.

That is, by adjusting the reaction conditions of the epoxy resin and theamine compound having at least one sulfur atom in a molecule, forexample, by setting the reaction conditions to a high temperature and along period of time, the viscosity of the obtained reaction product canbe increased, or by setting the reaction conditions to a low temperatureand a short period of time, the viscosity of the obtained reactionproduct can be controlled to be low. Therefore, the viscosity of thethermosetting resin composition can be adjusted by mixing the epoxyresin containing the reaction product having a desired viscosity in thethermosetting resin composition. In addition, by using the epoxy resinhaving the structure represented by Formula (1) for the thermosettingresin composition, rapid curability suitable for high cycle pressforming is easily obtained, and a cured product of the thermosettingresin composition easily obtains high mechanical properties.

It is preferable that the epoxy resin in the thermosetting resincomposition of the present invention contains the epoxy resin having thestructure represented by Formula (1) and a polyfunctional epoxy resinhaving three or more epoxy groups in a molecule. As the polyfunctionalepoxy resin having three or more epoxy groups in a molecule, apolyfunctional epoxy resin that does not have the structure representedby Formula (1) and contains three or more epoxy groups in a molecule ispreferable.

Examples of the polyfunctional epoxy resin having three or more epoxygroups in a molecule include a phenol novolac type epoxy resin, a cresolnovolac type epoxy resin, a trisphenolmethane type epoxy resin,tetraglycidyl diaminodiphenylmethanes, a glycidyl compound ofaminophenol, and a glycidyl compound of aminocresol.

Among these, a phenol novolac type epoxy resin, a trisphenolmethane typeepoxy resin, and a tetraglycidyl diaminodiphenylmethane are preferablefrom the viewpoint of exhibiting high heat resistance and highmechanical properties.

In a case where a material containing different numbers of epoxy groupsin the molecule, such as a phenol novolac type epoxy resin, iscontained, by fractionation through GPC, the amount of thepolyfunctional epoxy resin having three or more epoxy groups in amolecule is calculated.

The amount of the epoxy resin having the structure represented byFormula (1) is preferably 30 to 70 parts by mass, and more preferably 40to 65 parts by mass with respect to 100 parts by mass of the epoxy resinin the thermosetting resin composition.

When the amount of the epoxy resin having the structure represented byFormula (1) is equal to or more than the lower limit, rapid curabilityand high mechanical properties are easily obtained. When the amount ofthe epoxy resin having the structure represented by Formula (1) is equalto or less than the upper limit, a cured resin product having high heatresistance is easily obtained.

The amount of the polyfunctional epoxy resin having three or more epoxygroups in a molecule is preferably 20 to 40 parts by mass, and morepreferably 20 to 30 parts by mass with respect to 100 parts by mass ofthe epoxy resin in the thermosetting resin composition.

When the amount of the polyfunctional epoxy resin having three or moreepoxy groups in a molecule is equal to or more than the lower limit,high heat resistance is easily obtained. When the amount of thepolyfunctional epoxy resin having three or more epoxy groups in amolecule is equal to or less than the upper limit, high mechanicalproperties are easily obtained.

In addition, as will be described later, by using an epoxy resin that isliquid and an epoxy resin that is solid at room temperature incombination further with a thermoplastic resin as needed, the viscosityof the thermosetting resin composition can be adjusted to an optimalrange.

The amount of the epoxy resin that is liquid at room temperature withrespect to 100 parts by mass of the epoxy resin is preferably 10 to 90parts by mass, and more preferably 60 to 90 parts by mass.

By causing the amount of the epoxy resin that is liquid at roomtemperature with respect to 100 parts by mass of the epoxy resin to bewithin the above range, the viscosity of the thermosetting resincomposition can be set to an appropriate range.

Examples of commercial products of the epoxy resin that is liquid atroom temperature include: a bisphenol A type epoxy resin such as jER825, 826, 827, 828, and 834 (all manufactured by Mitsubishi ChemicalCorporation), EPICLON 850 (manufactured by DIC Corporation), EPOTOHTOYD-128 (NIPPON STEEL Chemical & Material Co., Ltd.), DER-331 and DER-332(manufactured by The Dow Chemical Company), and ARALDITE (registeredtrademark; the same applies hereinafter) LY556 (manufactured by HuntsmanAdvanced Materials);

a bisphenol F type epoxy resin such as jER 806, 807, and 1750(manufactured by Mitsubishi Chemical Corporation), EPICLON 830(manufactured by DIC Corporation), and EPOTOHTO YD-170 and EPOTOHTOYD-175 (manufactured by NIPPON STEEL Chemical & Material Co., Ltd.);a phenol novolac type epoxy resin such as jER 152 (manufactured byMitsubishi Chemical Corporation), EPICLON N-730A (manufactured by DICCorporation), and DEN-425 (manufactured by The Dow Chemical Company);an amine type epoxy resin such as jER 604 and 630 (both manufactured byMitsubishi Chemical Corporation), and MY 0600 and MY 0500 (bothmanufactured by Huntsman Advanced Materials); andan alicyclic epoxy resin such as CELLOXIDE 2021P and CELLOXIDE 8000(manufactured by Daicel Corporation).

One of these epoxy resins that are liquid at room temperature may beused alone, or two or more thereof may be used in combination.

As the epoxy resin that is liquid at room temperature, a bisphenol Atype epoxy resin and a phenol novolac type epoxy resin are preferablefrom the viewpoint of excellent balance between toughness and heatresistance of a cured product.

The amount of the epoxy resin that is solid at room temperature withrespect to 100 parts by mass of the epoxy resin is preferably 10 to 90parts by mass, and more preferably 10 to 30 parts by mass.

By causing the amount of the epoxy resin that is solid at roomtemperature with respect to 100 parts by mass of the epoxy resin to bewithin the above range, the viscosity of the thermosetting resincomposition can be set to an appropriate range.

Examples of commercial products of the epoxy resin that is solid at roomtemperature include: a phenol novolac type epoxy resin such as jER 154and 157S70 (both manufactured by Mitsubishi Chemical Corporation), andEPICLON N-770, EPICLON N-740, and EPICLON N-775 (all manufactured by DICCorporation); a cresol novolac type epoxy resin such as EPICLON N-660,EPICLON N-665, EPICLON N-670, EPICLON N-673, and EPICLON N-695 (allmanufactured by DIC Corporation), and EOCN (registered trademark; thesame applies hereinafter)-1020, EOCN-102S, and EOCN-104S (allmanufactured by Nippon Kayaku Co., Ltd.); a bisphenol A type epoxy resinsuch as jER 1001, 1002, and 1003 (all manufactured by MitsubishiChemical Corporation);

a bisphenol F type epoxy resin such as jER 4004P and 4005P (bothmanufactured by Mitsubishi Chemical Corporation);

a trisphenolmethane type epoxy resin such as jER 1032H60 (manufacturedby Mitsubishi Chemical Corporation);

a biphenyl type epoxy resin such as YX4000 and YL6121H (bothmanufactured by Mitsubishi Chemical Corporation);

a naphthalene type epoxy resin such as HP4700 (manufactured by DICCorporation);

a dicyclopentadiene type epoxy resin such as HP 7200 (manufactured byDIC Corporation);

an epoxy resin having an oxazolidone ring skeleton, such as TSR-400(manufactured by DIC Corporation), DER 858 (manufactured by The DowChemical Company), and AER 4152 (manufactured by Asahi Kasei E-MaterialsCo.); and

a bisphenol S type epoxy resin such as EXA-1514 and EXA-1517(manufactured by DIC Corporation).

In addition, as the epoxy resin that is solid at room temperature, anepoxy resin having the structure represented by Formula (1) in amolecule may be used.

By using the epoxy resin having the structure represented by Formula (1)in a molecule as the epoxy resin that is solid at room temperature, thecuring time of the thermosetting resin composition can be easilyreduced, and furthermore, a cured product of the thermosetting resincomposition can easily obtain high mechanical properties.

One of these epoxy resins that are solid at room temperature may be usedalone, or two or more thereof may be used in combination.

The molecular weight of the epoxy resin in the thermosetting resincomposition of the present invention is preferably 200 to 3000, and morepreferably 300 to 2000. When the molecular weight of the epoxy resin iswithin the above range, the thermosetting resin composition can beeasily adjusted to a desired viscosity.

The molecular weight is a weight average molecular weight in terms ofpolystyrene measured by gel permeation chromatography.

<Molecular Weight Fractionation Conditions>

Under the following conditions, each component of the epoxy resin isfractionated and the molecular weight of the epoxy resin is calculated.

The weight average molecular weight of each fraction collected is takenas the molecular weight of the fraction, and the mass ratio of thefraction is calculated from the ratio of the peak area to the total peakarea.

Equipment: HLC-8020 (component system) (manufactured by TosohCorporation)

Pump: DP-8020 (two computer-controlled dual pumps)

Online degasser: SD-8022

Autosampler: AS-8020

RI detector: RI-8020

UV detector: UV-8020

Fraction collector: FC-8020

Column: TSK-GEL G3000 (21.5 mm ID×60 cmL)×2+Guard column attached

Flow rate: 4.0 ml/min (2.0 ml/min×2)

Mobile phase: chloroform

Column temperature: room temperature

Injection volume: 1.0 ml

Concentration: 6 mass %

The epoxy equivalent weight of the epoxy resin in the thermosettingresin composition of the present invention is preferably 120 to 300g/eq, and more preferably 150 to 280 g/eq.

When the epoxy equivalent weight of the epoxy resin is equal to or morethan the lower limit, the resin does not become too brittle, and highmechanical properties are easily obtained. When the epoxy equivalentweight of the epoxy resin is equal to or less than the upper limit, theheat resistance hardly decreases.

The epoxy equivalent weight means the molecular weight of the epoxyresin per epoxy group.

The amount of the epoxy resin in 100 parts by mass of the thermosettingresin composition of the present invention is preferably 60 to 95 partsby mass, more preferably 65 to 93 parts by mass, and even morepreferably 70 to 90 parts by mass.

When the amount of the epoxy resin is equal to or more than the lowerlimit, mechanical properties are easily kept high. When the amount ofthe epoxy resin is equal to or less than the upper limit, the heatresistance at the time of curing hardly decreases.

[Epoxy Resin Curing Agent]

The epoxy resin curing agent in the thermosetting resin composition ofthe present invention is dicyandiamide or a derivative thereof.

Dicyandiamide and a derivative thereof have a high melting point, andthe compatibility with the epoxy resin is suppressed in a lowtemperature range. In addition, by causing the epoxy resin compositionto contain dicyandiamide or a derivative thereof, excellent pot life isobtained, and the mechanical properties of the cured resin product areeasily improved.

Examples of the derivative of dicyandiamide include those in whichdicyandiamide is bonded to various compounds such as an epoxy resin, avinyl compound, an acrylic compound, and9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide.

As the epoxy resin curing agent in the thermosetting resin compositionof the present invention, one of these may be used alone, or two or morethereof may be used in combination. Moreover, dicyandiamide may be usedin combination.

As the epoxy resin curing agent in the thermosetting resin compositionof the present invention, dicyandiamide is preferable from the viewpointof reactivity.

Commercial products may be used as dicyandiamide or a derivativethereof.

Examples of commercial products of dicyandiamide include DICY 7 and DICY15 (both manufactured by Mitsubishi Chemical Corporation) and Dicyanex1400F (manufactured by Evonic Corporation), but are not limited thereto.

The amount of dicyandiamide or a derivative thereof in 100 parts by massof the thermosetting resin composition of the present invention ispreferably 1 to 9 parts by mass, more preferably 3 to 8 parts by mass,and even more preferably 3.5 to 7 parts by mass.

When the amount of dicyandiamide or a derivative thereof is equal to ormore than the lower limit, the epoxy resin contained in the epoxy resincomposition can be sufficiently cured. When the amount of dicyandiamideor a derivative thereof is equal to or less than the upper limit,storage stability is easily secured, and the toughness of the curedresin product is easily increased.

[Imidazole Compound]

The imidazole compound in the thermosetting resin composition of thepresent invention is a compound having an imidazole ring in a molecule,and is preferably at least one selected from the group consisting of acompound having a substituent on the imidazole ring (hereinafter, alsoreferred to as “imidazole derivative”), a compound having a structurewith imidazole or an imidazole derivative added to an epoxy group of anepoxy resin by ring opening (hereinafter, also referred to as “imidazoleadduct”), a compound in which imidazole or an imidazole derivative isincluded by a different molecule (hereinafter, also referred to as“inclusion compound of imidazole”), a microencapsulated imidazole orimidazole derivative (hereinafter, also referred to as“microencapsulated imidazole”), and imidazole or an imidazole derivativecoordinated with a stabilizer or the like (hereinafter, also referred toas “addition product comprising imidazole”).

Among these, as the imidazole compound in the thermosetting resincomposition of the present invention, an imidazole compound having acuring start temperature of 100° C. or higher is preferably used.

The imidazole compound having a curing start temperature of 100° C. orhigher has low reactivity at a relatively low temperature such as roomtemperature, and an epoxy resin composition containing the imidazolecompound having a curing start temperature of 100° C. or higher has highthermal stability. Therefore, the epoxy resin composition containing theimidazole compound having a curing start temperature of 100° C. orhigher and a prepreg containing the same have high storage stability. Onthe other hand, at a forming temperature of the epoxy resin composition,high curability and curing acceleration are exhibited.

The curing start temperature of the imidazole compound in thethermosetting resin composition of the present invention is morepreferably 110° C. or higher.

The curing start temperature is a value measured by the followingmethod.

<Curing Start Temperature of Imidazole Compound>

A curing start temperature of an imidazole compound is a temperature atan intersection between a tangent at an inflection point and a baselinein a DSC curve which is obtained by measuring a caorific value by adifferential scanning calorimeter (DSC) at a temperature rising rate of10° C./min of a sample resin composition prepared by mixing 10 parts bymass of the imidazole compound with 100 parts by mass of a bisphenol Atype epoxy resin having an epoxy equivalent weight of 180 to 220.

As the imidazole compound having a curing start temperature of 100° C.or higher, there are an imidazole derivative, an imidazole adduct, aninclusion compound of imidazole, a microencapsulated imidazole, anaddition product comprising imidazole, and the like.

One of these imidazole compounds may be used alone, or two or morethereof may be used in combination.

Examples of the imidazole derivative having a curing start temperatureof 100° C. or higher include2,4-diamino-6-[2′-methylimidazolyl-(1′)]-ethyl-s-triazine,1-cyanoethyl-2-phenylimidazoliumtrimellitate,2-phenyl-4,5-dihydroxymethylimidazole, and2-phenyl-4-methyl-5-hydroxymethylimidazole.

Specific examples of the imidazole derivative before the adducttreatment, the inclusion treatment with a different molecule, and themicroencapsulation treatment include 2-methylimidazole,2-ethyl-4-methylimidazole, 2-undecylimidazole, 2-heptadecylimidazole,1,2-dimethylimidazole, 2-phenylimidazole, 2-phenyl-4-methylimidazole,1-benzyl-2-phenylimidazole, 1-benzyl-2-methylimidazole,1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole,1-cyanoethyl-2-undecylimidazole, 1-cyanoethyl-2-phenylimidazole,1-cyanoethyl-2-ethyl-4-methylimidazoliumtrimellitate,1-cyanoethyl-2-undecylimidazoliumtrimellitate,1-cyanoethyl-2-phenylimidazoliumtrimellitate,2,4-diamino-6-(2′-methylimidazolyl-(1′))ethyl-s-triazine,2,4-diamino-6-(2′-undecylimidazolyl-(1′))-ethyl-s-triazine,2,4-diamino-6-(2′-ethyl-4-methylimidazolyl-(1′))-ethyl-s-triazine,1-cyanoethyl-2-phenyl-4,5-di(2-cyanoethoxy)methylimidazole,2-phenyl-4,5-dihydroxymethylimidazole, and2-phenyl-4-methyl-5-hydroxymethylimidazole, but are not limited thereto.

Examples of the addition product comprising imidazole having a curingstart temperature of 100° C. or higher include imidazole or an imidazolederivative coordinated with a stabilizer.

Preferable examples of the addition product comprising imidazole includethose in which imidazole or an imidazole derivative is coordinated to aboric acid compound, isocyanuric acid, a metal atom, or the like tocontrol the reactivity of the imidazole ring and the epoxy group.

Among these, an isocyanuric acid addition product of imidazole or anisocyanuric acid addition product of an imidazole derivative ispreferable because stability in the epoxy resin is easily secured andthe heat resistance of the cured product with the epoxy resin is hardlyreduced.

Specific examples of the imidazole derivative before the addition ofisocyanuric acid include 2-methylimidazole, 2-ethyl-4-methylimidazole,2-undecylimidazole, 2-heptadecylimidazole, 1,2-dimethylimidazole,2-phenylimidazole, 2-phenyl-4-methylimidazole,1-benzyl-2-phenylimidazole, 1-benzyl-2-methylimidazole,1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole,1-cyanoethyl-2-undecylimidazole, 1-cyanoethyl-2-phenylimidazole,1-cyanoethyl-2-ethyl-4-methylimidazoliumtrimellitate,1-cyanoethyl-2-undecylimidazoliumtrimellitate,1-cyanoethyl-2-phenylimidazoliumtrimellitate,2,4-diamino-6-(2′-methylimidazolyl-(1′))ethyl-s-triazine,2,4-diamino-6-(2′-undecylimidazolyl-(1′))-ethyl-s-triazine,2,4-diamino-6-(2′-ethyl-4-methylimidazolyl-(1′))-ethyl-s-triazine,1-cyanoethyl-2-phenyl-4,5-di(2-cyanoethoxy)methylimidazole,2-phenyl-4,5-dihydroxymethylimidazole, and2-phenyl-4-methyl-5-hydroxymethylimidazole, but are not limited thereto.

In particular, because of their high stability and high curability,2,4-diamino-6-(2′-methylimidazolyl-(1′))-ethyl-s-triazine/isocyanuricacid addition product, 2-phenylimidazole/isocyanuric acid additionproduct, 2-methylimidazole/isocyanuric acid addition product,2-phenyl-4,5-dihydroxymethylimidazole/isocyanuric acid addition product,and 2-phenyl-4-methyl-5-hydroxymethylimidazole/isocyanuric acid additionproduct are preferable as the addition product comprising imidazolehaving a curing start temperature of 100° C. or higher.

From the viewpoint of more easily improving the storage stability, as amore preferable addition product comprising imidazole having a curingstart temperature of 100° C. or higher, there is an isocyanuric acidaddition product of an imidazole derivative having a triazine skeleton(triazine ring) in a molecule.

Examples of the isocyanuric acid addition product of an imidazolederivative having a triazine skeleton (triazine ring) in a moleculeinclude2,4-diamino-6-(2′-methylimidazolyl-(1′))-ethyl-s-triazine/isocyanuricacid addition product,1-(4,6-diamino-s-triazine-2-yl)ethyl-2-undecylimidazole/isocyanuric acidaddition product, and2,4-diamino-6-[2-(2-ethyl-4-methyl-1-imidazolyl)ethyl]-s-triazine/isocyanuricacid addition product.

From the viewpoint of easily obtaining storage stability as a prepregand rapid curability suitable for high cycle press forming,2,4-diamino-6-(2′-methylimidazolyl-(1′))-ethyl-s-triazine/isocyanuricacid addition product is particularly preferable.

Examples of commercial products of the2,4-diamino-6-(2′-methylimidazolyl-(1))-ethyl-s-triazine/isocyanuricacid addition product include 2MA-OK (manufactured by Shikoku ChemicalsCorporation), but are not limited thereto.

Among these imidazole compounds having a curing start temperature of100° C. or higher, an addition product comprising imidazole having acuring start temperature of 100° C. or higher is preferable from theviewpoint of easily securing stability and curability.

The imidazole compound in the thermosetting resin composition of thepresent invention is mixed in the thermosetting resin composition withdicyandiamide or a derivative thereof and the urea derivative having twoor more dimethylureido groups in a molecule, thereby easily curing thethermosetting resin composition within a short period of time.Furthermore, by causing the thermosetting resin composition of thepresent invention to contain the imidazole compound, the heat resistanceof the cured product is easily improved.

The amount of the imidazole compound in 100 parts by mass of thethermosetting resin composition of the present invention is preferably 1to 8 parts by mass, more preferably 1 to 7 parts by mass, even morepreferably 1 to 4 parts by mass, and particularly preferably 1.5 to 3parts by mass.

When the amount of the imidazole compound is equal to or more than thelower limit, the curability of the thermosetting resin composition iseasily improved, and the cured product tends to exhibit high heatresistance. When the amount of the imidazole compound is equal to orless than the upper limit, storage stability is hardly impaired, andmechanical properties are easily kept high.

Usually, an imidazole compound is a crystalline solid under thecondition of room temperature (25° C.), and has low solubility in anepoxy resin at 100° C. or lower. Therefore, from the viewpoint of beingfavorably dispersed in the thermosetting resin composition andaccelerating the curing reaction, the imidazole compound is preferably apowder having a volume average particle size of preferably 100 μm orless, and more preferably 20 μm or less.

The volume average particle size can be measured with a particle sizeanalyzer (manufactured by Nikkiso Co., Ltd., product name: AEROTRAC SPRModel: 7340), and is taken as the value of D50 of the measured particlesize distribution.

[Epoxy Resin Curing Accelerator]

The epoxy resin curing accelerator in the thermosetting resincomposition of the present invention contains a urea derivative havingtwo or more dimethylureido groups in a molecule.

The urea derivative having two or more dimethylureido groups in amolecule is not particularly limited as long as the urea derivative hastwo or more dimethylureido groups in a molecule and generates anisocyanate group and dimethylamine by being heated at a hightemperature, and those generated activate the epoxy group of the epoxyresin and dicyandiamide or a derivative thereof. In a case where onlyone dimethylureido group is included in a molecule, a decrease in heatresistance due to a decrease in crosslink density occurs in the reactionwith the epoxy resin.

Examples of the urea derivative include aromatic dimethylurea in which adimethylureido group is bonded to an aromatic ring, and aliphaticdimethylurea in which a dimethylureido group is bonded to an aliphaticcompound.

Among these, aromatic dimethylurea is preferable in that the curing rateis increased.

Examples of the aromatic dimethylurea having two or more dimethylureidogroups in a molecule include methylene bis(phenyldimethylurea) andtolylene bis(dimethylurea). Specific examples thereof include4,4′-methylene bis(phenyldimethylurea) (MBPDMU),2,4-bis(3,3-dimethylureido)toluene (TBDMU), and 2,4-tolylenebis(N′-2,4,6-tribromophenylurea).

Among these, TBDMU is preferable because the curing accelerating abilityis high and heat resistance can be easily imparted to the cured resinproduct.

These aromatic dimethylureas having two or more dimethylureido groups ina molecule may be used alone or two or more thereof may be used incombination.

Examples of the aliphatic dimethylurea having two or more dimethylureidogroups in a molecule include dimethylurea obtained from isophoronediisocyanate and dimethylamine, dimethylurea obtained from m-xylylenediisocyanate and dimethylamine, and dimethylurea obtained fromhexamethylene diisocyanate and dimethylamine.

As the urea derivative having two or more dimethylureido groups in amolecule, commercial products may be used.

Examples of commercial products of MBPDMU include Technicure MDU-11(manufactured by A&C Catalysts, Inc.), and Omicure (registeredtrademark; the same applies hereinafter) (Omicure) 52 (manufactured byPTI Japan, Ltd.), but are not limited thereto.

Examples of commercial products of TBDMU include Omicure (Omicure) 24(manufactured by PTI Japan, Ltd.), but are not limited thereto.

The amount of the urea derivative having two or more dimethylureidogroups in a molecule in 100 parts by mass of the thermosetting resincomposition of the present invention is preferably 1 to 8 parts by mass,more preferably 1 to 5 parts by mass, and even more preferably 1.5 to 4parts by mass.

When the amount of the urea derivative having two or more dimethylureidogroups in a molecule is equal to or more than the lower limit, thecuring accelerating action of the epoxy resin contained in the epoxyresin composition can be easily sufficiently obtained. When the amountof the urea derivative having two or more dimethylureido groups in amolecule is equal to or less than the upper limit, a cured resin productexcellent in storage stability, heat resistance, and mechanicalproperties is easily obtained.

Among the urea derivatives having two or more dimethylureido groups in amolecule, MBPDMU and TBDMU are crystalline solids under the condition ofroom temperature (25° C.), and have low solubility in an epoxy resin at100° C. or lower. Therefore, from the viewpoint of being favorablydispersed in the thermosetting resin composition and accelerating thecuring reaction, the urea derivative having two or more dimethylureidogroups in a molecule is preferably a powder having a volume averageparticle size of preferably 100 μm or less, and more preferably 20 μm orless.

The volume average particle size can be measured in the same manner asthe volume average particle size of the imidazole compound.

[Optional Components]

The thermosetting resin composition of the present invention may containoptional components that do not correspond to any of dicyandiamide or aderivative thereof, the imidazole compound, and the urea derivativehaving two or more dimethylureido groups in a molecule, within a rangethat does not impair the gist of the present invention.

However, as the optional components, other curing agents that areexcellent in curability at low temperatures need to be added only in asmall amount because the life of a resin film containing thethermosetting resin composition, that is, the period of time for whichthe resin film can be stored while holding its tackiness andflexibility, is shortened.

Furthermore, the thermosetting resin composition of the presentinvention may contain a thermoplastic resin in order to adjust theviscosity to an optimal range.

As the thermoplastic resin, a polyvinyl acetal resin, a polyvinylalcohol resin, a phenoxy resin, a polysulfone, or a polyethersulfone,which has high solubility in an epoxy resin, is preferable.

Examples of commercial products of the thermoplastic resin include:DENKA BUTYRAL (manufactured by Denka Company Limited) as the polyvinylacetal resin; DENKA POVAL (registered trademark; the same applieshereinafter) (manufactured by Denka Company Limited) and VINYLEC(registered trademark; the same applies hereinafter) (manufactured byChisso Corporation) as the polyvinyl alcohol resin; YP-50 (manufacturedby NIPPON STEEL Chemical & Material Co., Ltd.), YP-50s (manufactured byNIPPON STEEL Chemical & Material Co., Ltd.), and YP-70 (manufactured byNIPPON STEEL Chemical & Material Co., Ltd.) as the phenoxy resin; UDEL(registered trademark; the same applies hereinafter) (manufactured bySolvay Advanced Polymers) as the polysulfone; and Ultrason (registeredtrademark; the same applies hereinafter) (manufactured by BASF) as thepolyethersulfone.

The amount of the thermoplastic resin in the thermosetting resincomposition of the present invention is preferably 1 to 20 parts bymass, and more preferably 2 to 10 parts by mass, with respect to 100parts by mass of the epoxy resin in the thermosetting resin composition.

When the amount of the thermoplastic resin is equal to or more than thelower limit, the effect of reducing tack at room temperature andsuppressing resin flow during curing is easily exhibited. When theamount of the thermoplastic resin is equal to or less than the upperlimit, the mechanical properties of the cured product are not easilyimpaired.

Furthermore, the thermosetting resin composition of the presentinvention may contain various additives, resins, fillers, and the likewithin a range that does not impair the gist of the present invention.

[Curing Completion Time of Thermosetting Resin Composition]

The curing completion time of the thermosetting resin composition of thepresent invention measured with a curelastometer is preferably 6 minutesor shorter, and more preferably 5 minutes or shorter at 140° C.

When curing is completed within the above-mentioned time, it can be saidthat rapid curability as a resin composition used for high cycle pressforming is sufficiently provided.

The above properties can be achieved by using the imidazole compound andthe urea derivative having two or more dimethylureido groups in amolecule in combination. By using an epoxy resin having a structurerepresented by Formula (1), rapid curability can be further improved.

The curelastometer applies a sinusoidal vibration with a predeterminedamplitude that does not destroy the resin composition at a predeterminedtemperature to the resin composition, continuously measures the torquetransmitted from the resin composition to an upper die, and records achange in viscoelastic stress during the curing reaction as a torqueamplitude/time curve (cure curve).

For the curing completion time of the thermosetting resin composition ofthe present invention, a maximum torque value (Tmax) at which the torquevalue changes no longer is obtained from the cure curve, and a torquevalue of 90% of the maximum torque value (T90) is calculated. The timeneeded until T90 is reached after the start of the measurement isobtained, and this is taken as the curing completion time (t90).

[Viscosity of Thermosetting Resin Composition]

In the thermosetting resin composition of the present invention, whenthe viscosity after holding at 60° C. for 5 minutes is V0 and theviscosity after holding at 60° C. for 5 hours is V1, the viscosity ratioV1/V0 is preferably 1.0 to 4.5, more preferably 1.0 to 2.5, and evenmore preferably 1.0 to 2.0.

By causing the viscosity ratio of the thermosetting resin composition ofthe present invention to be within the above range, it is easy toprevent an increase in viscosity during the preparation of a resin orthe production of a prepreg, which causes a difficulty in obtaining agood prepreg. In a case where an increase in viscosity is incurredduring the preparation of a resin or during the production of a prepreg,the tack of the prepreg is lost and lamination cannot be achieved, orthe prepreg cannot be sufficiently impregnated with the resin, so thatvoids remain inside the fiber-reinforced composite material or thesmoothness of the surface is impaired, resulting in deterioration ofappearance.

In general, the above properties can be achieved by adjusting the amountof the curing agent in the epoxy resin. However, in a case where theamount of the curing agent is reduced to cause the viscosity ratio V1/V0to fall within the above range, the rapid curability is easily lost dueto the decrease in the amount of the curing agent. That is, there arecases where it is difficult to maintain the rapid curability and tocause the viscosity ratio V1/V0 to fall within the above range bychanging the amount of the curing agent.

On the other hand, by also using the imidazole compound, it is possibleto achieve both rapid curability and the above-mentioned properties.From the viewpoint of improving the storage stability and rapidcurability of the thermosetting resin composition, the imidazolecompound preferably contains one selected from the group consisting ofan imidazole derivative, an imidazole adduct, an inclusion compound ofimidazole, a microencapsulated imidazole, and an addition productcomprising imidazole. Among these, an addition product comprisingimidazole is particularly preferable from the viewpoint of improvingstorage stability.

The viscosity at 30° C. of the thermosetting resin composition of thepresent invention is preferably 1.0×10² to 1.0×10⁵ Pa·s, more preferably5.0×10² to 9.8×10⁴ Pa·s, and even more preferably 1.0×10³ to 9.7×10⁴Pa·s.

When the viscosity of the thermosetting resin composition is equal to ormore than the lower limit, the handleability of a resin film is likelyto be excellent, and operations such as production, lamination, andforming of the resin film are facilitated. When the viscosity of thethermosetting resin composition is equal to or less than the upperlimit, for example, during the production of a resin film containing areinforcing fiber base material, which will be described later, thereinforcing fiber base material can be easily impregnated with thethermosetting resin composition, excessive heating is not necessaryduring the impregnation, and the drapability of the resin film is noteasily impaired.

The viscosity of the thermosetting resin composition can be caused tofall within the above range by melt-mixing a solid thermosetting resinand a liquid thermosetting resin.

In the thermosetting resin composition of the present invention, in aviscosity measurement in which the initial temperature is 30° C. and thetemperature is raised at 2.0° C./min, the temperature at which thelowest viscosity is obtained is preferably 100° C. to 130° C. Inaddition, the minimum viscosity in this case is preferably 0.1 Pa·s to50 Pa·s, and more preferably from 0.5 Pa·s to 10 Pa·s.

When the temperature at which the minimum viscosity is obtained iswithin the above range, the flow rate of the thermosetting resincomposition during forming can be easily suppressed to an appropriaterange, and a formed article having excellent appearance is easilyobtained. When the minimum viscosity in this case is within the aboverange, a formed article having good appearance is easily obtained. Inparticular, when the minimum viscosity is equal to or more than thelower limit, excessive flow is easily suppressed, and appearance defectssuch as irregularities on the surface of the formed article are lesslikely to occur. When the minimum viscosity is equal to or less than theupper limit, the thermosetting resin composition spreads to every cornerof the die at the time of forming, and a formed article having goodappearance is easily obtained.

The temperature at which the minimum viscosity is obtained can be setwithin the above range by selecting the type of the curing agent. Theminimum viscosity in this case can be set within the above range bymelt-mixing a solid thermosetting resin, a liquid thermosetting resin, athermoplastic resin, and the like.

[Production Method for Thermosetting Resin Composition]

The thermosetting resin composition of the present invention can beproduced by a known method. For example, a glass flask, a kneader, aplanetary mixer, a general stirring and heating pot, or a stirring andpressure heating pot is preferably used for the production.

The thermosetting resin composition of the present invention ispreferably produced by, for example, a production method having thefollowing steps.

Step (1): a step in which an epoxy resin and an additive such as athermoplastic resin are put into a dissolving container and heated andmixed at 70° C. to 150° C. for 1 to 6 hours, thereby obtaining an epoxyresin main ingredient.

Step (2): a step in which, after cooling the epoxy resin main ingredientto 50° C. to 70° C., an epoxy resin curing agent, an imidazole compound,and an epoxy resin curing accelerator are added and mixed at 50° C. to70° C. for 0.5 to 2 hours, thereby obtaining an epoxy resin composition.

<<Prepreg>>

A prepreg of the present invention includes the thermosetting resincomposition of the present invention and a reinforcing fiber basematerial. Specifically, the prepreg is a sheet-like prepreg having areinforcing fiber base material impregnated with the thermosetting resincomposition of the present invention.

The prepreg is used as a single layer or as a laminated body obtained bylaminating a plurality of layers as necessary. For example, asillustrated in FIG. 1 , a prepreg laminate 1 in which six prepregs 10are laminated is used.

Reinforcing fibers constituting the reinforcing fiber base material arenot particularly limited, and for example, inorganic fibers, organicfibers, metal fibers, or reinforcing fibers having a hybridconfiguration of a combination thereof can be used.

Examples of the inorganic fibers include carbon fibers, graphite fibers,silicon carbide fibers, alumina fibers, tungsten carbide fibers, boronfibers, and glass fibers.

Examples of the organic fibers include aramid fibers, high-densitypolyethylene fibers, other general nylon fibers, and polyester fibers.

Examples of the metal fibers include fibers of stainless steel, iron,and the like, and carbon fibers coated with metal.

Among these, carbon fibers are preferable in consideration of mechanicalproperties such as the strength of the fiber-reinforced compositematerial.

The reinforcing fibers of the reinforcing fiber base material may belong fibers or short fibers. Long fibers are preferred because of theirexcellent rigidity.

Examples of the form of the reinforcing fiber base material include aform in which a large number of long fibers are aligned in one directionto form a UD sheet (unidirectional sheet), a form in which long fibersare woven into a cloth material (fabric), and a form in which shortfibers form a nonwoven fabric.

Examples of a method of weaving the cloth material include plain weave,twill weave, satin weave, and triaxial weave.

The fiber basis weight of the reinforcing fiber base material ispreferably 50 to 800 g/m², and more preferably 75 to 300 g/m².

When the fiber basis weight of the reinforcing fiber base material isequal to or more than the lower limit, the number of laminated layersneeded to obtain a formed article having a desired thickness can bereduced. When the fiber basis weight of the reinforcing fiber basematerial is equal to or less than the upper limit, a prepreg basematerial in a favorable impregnated state is easily obtained.

The lamination configuration of the prepreg base material is notparticularly limited.

For example, in a case of using UD prepreg base materials, aconfiguration in which the UD prepreg base materials are laminated tocause the fiber axes of the reinforcing fibers of the UD prepreg basematerials vertically adjacent to be orthogonal to each other isexemplified.

Regarding the prepreg base materials, only the same type of prepreg basematerial may be laminated, or different types of prepreg base materialsmay be laminated.

The number of layers of the prepreg base materials is not particularlylimited, and can be appropriately determined according to the requiredcharacteristics of the fiber-reinforced composite material and the like.

<<Fiber-Reinforced Composite Material>>

A fiber-reinforced composite material of the present invention is acured product of the prepreg of the present invention.

The fiber-reinforced composite material is obtained by subjecting theprepreg to heating and pressurization forming.

[Forming Step]

A prepreg laminate obtained by laminating a required number of prepregsis subjected to heating and pressurization forming by a die to obtain afiber-reinforced composite material.

As a heating and pressurization forming method for the prepreg laminateusing the die, a known heating and pressurization forming method can beadopted, and examples thereof include autoclave forming, oven forming,internal pressure forming, and press forming.

Although the press forming facilitates obtaining a fiber-reinforcedcomposite material having a resin layer formed from a resin film on thesurface layer compared to other heating and pressurization formingmethods, the forming pressure is high and the resin tends to flow out ofthe die. However, since the thermosetting resin composition of thepresent invention has excellent rapid curability, the outflow of theresin from the die during the forming can be suppressed. Therefore, thethermosetting resin composition of the present invention is moreeffective in a case where press forming is adopted in the forming step,and is particularly effective in a case where high cycle press formingis adopted.

For example, a case where a prepreg laminate 1 is press-formed by a die100 illustrated in FIG. 2 will be described.

The die 100 includes a lower die 110 provided with a protruding portion112 on the upper surface side and an upper die 120 provided with arecessed portion 122 on the lower surface side. When the upper die 120is brought close to the lower die 110 and the die 100 is closed, acavity complementary in shape to the shape of a target fiber-reinforcedcomposite material is formed between the protruding portion 112 and therecessed portion 122 in the die 100.

After disposing the prepreg laminate 1 on the lower die 110 asillustrated in (a) of FIG. 2 , the upper die 120 is brought close to thelower die 110 to close the die 100 as illustrated in (b) of FIG. 2 , andthe prepreg laminate 1 is subjected to heating and pressurizationforming. The thermosetting resin composition in the prepreg laminate 1is cured while flowing by being pressed by the die 100 and heated.

After the curing, as illustrated in (c) of FIG. 2 , the die 100 isopened to take out the fiber-reinforced composite material 2, whereby afiber-reinforced composite material as illustrated in FIG. 3 isobtained.

As the heating and pressurization forming conditions, known heating andpressurization forming conditions can be adopted except that the prepregof the present invention (prepreg laminate 1) is used.

The die temperature during heating and pressurization forming ispreferably 100° C. to 180° C., and more preferably 120° C. to 160° C.

Through heating at the lower limit or higher, rapid curing can beachieved and the forming cycle can be shortened. Through heating at theupper limit or lower, the resin flow during the forming is suppressed,and a formed article having good appearance can be obtained.

The surface pressure during the heating and pressurization forming ispreferably 1 to 15 MPa, and more preferably 4 to 10 MPa.

By applying a pressure equal to or more than the lower limit, the resinflows and the resin composition spreads to every corner of the die, sothat a formed article having good appearance is easily obtained. Byapplying a pressure equal to or less than the upper limit, it ispossible to prevent deterioration of the formed appearance due toexcessive flow of the resin.

The heating and pressurization forming time is preferably 1 to 15minutes, more preferably 2 to 8 minutes, and even more preferably 2 to 5minutes.

Forming for a time equal to or longer than the lower limit enables theproduction of a resin composition having excellent storage stability andrapid curability. Forming for a time equal to or shorter than the upperlimit enables high cycle press forming.

[Shaping Step]

A production method for the fiber-reinforced composite material of thepresent invention may further include a shaping step of producing apreform having a near net shape of a desired formed article shape byshaping one sheet or two or more overlapping sheets of the prepreg ofthe present invention. That is, the production method for thefiber-reinforced composite material of the present invention may be amethod of performing a laminating step of overlapping two or more sheetsof the prepreg of the present invention as necessary, the shaping step,and the forming step in this order.

In this case, the preform having a near net shape of a desired formedarticle shape is subjected to heating and pressurization forming in theforming step to produce a fiber-reinforced composite material having thedesired formed article shape.

A shaping method for the prepreg laminate may be any method as long asthe prepreg laminate can be shaped into an intermediate shape based onthe shape of a target fiber-reinforced composite material, and a knownmethod can be adopted except that one sheet or two or more overlappingsheets of the prepreg of the present invention are used.

(Heat Resistance)

The glass transition temperature of the fiber-reinforced compositematerial of the present invention obtained by dynamic viscoelasticitymeasurement is preferably 135° C. or higher, and more preferably 150° C.or higher.

When the glass transition temperature of the fiber-reinforced compositematerial is equal to or higher than the lower limit, thefiber-reinforced composite material can be used for a portion requiringhigh heat resistance.

The glass transition temperature of the fiber-reinforced compositematerial of the present invention by dynamic viscoelasticity measurementis determined, for example, by the temperature dependence of the storagerigidity (G′) obtained by dynamic viscoelasticity measurement of a curedproduct obtained by placing a prepreg in a die preheated to 140° C.,closing the die, and holding the prepreg at 140° C. for 5 minutes whileapplying a pressure thereto.

EXAMPLES

Hereinafter, the present invention will be described in detail withreference to examples, but the present invention is not limited to thefollowing description.

[Isothermal Viscosity Stability: Measurement of Viscosities V0 and V1 ofThermosetting Resin Composition]

The viscosity of the thermosetting resin composition was measured underthe following conditions.

Equipment: Rheometer (manufactured by Thermo Fisher Scientific, “MARS40”)

Plate used: 25φ parallel plate

Plate gap: 0.5 mm

Measurement frequency: 10 rad/sec

Measurement temperature: 60° C.

Measurement time: 5 hours

Stress: 300 Pa

The viscosity measurement was started by the above measurement method,the viscosity after 5 minutes was referred to as V0, and the viscosityafter 5 hours was referred to as V1.

[Evaluation of 60° C. Isothermal Viscosity Stability]

Evaluation was made from the ratio between the viscosities V0 and V1according to the following evaluation criteria.

(Evaluation Criteria)

AA: V0/V1 is within 1.0 to 2.0. There is no significant change inviscosity during the preparation of a resin composition and during theproduction of a prepreg, and a good prepreg can be produced.

A: V0/V1 is within 2.0 to 2.5.

B: V0/V1 is within 2.5 to 4.5.

C: V0/V1 is 4.5 or more. There is a significant change in viscosityduring the preparation of a resin composition and during the productionof a prepreg, and a good prepreg cannot be produced.

[Curing Time of Curelastometer: Curing Completion Time of ThermosettingResin Composition]

The curing completion time of the thermosetting resin composition (thetime to reach 90% torque of the maximum torque value) was obtained asfollows.

A change in the torque value (N·m) at a die temperature of 140° C. wasmeasured using “CURELASTOMETER 7 Type P” manufactured by JSR TradingCo., Ltd. Next, a maximum torque value (Tmax) at which the torque valuechanges no longer is determined from a torque value change curve, and atorque value of 90% of the maximum torque value (T90) is calculated. Thetime needed until T90 is reached after the start of the measurement wasobtained, and this was taken as the curing completion time (t90). Themeasurement conditions are shown below.

Equipment: Curelastometer (CURELASTOMETER 7 Type P, manufactured by JSRTrading Co., Ltd.)

Die effective bore diameter: φ160 mm

Measurement temperature: 140° C.

[Measurement of Dynamic Viscoelasticity of Cured Product (GlassTransition Temperature)]

A formed plate obtained in each of the examples and the comparativeexamples was processed into a test piece having a length of 55 mm and awidth of 12.5 mm, and was measured in a torsion mode using RheometerARES-RDA manufactured by TA Instruments. At a measurement frequency of 1Hz and a temperature rising rate of 5° C./min, log G′ was plottedagainst temperature, and the temperature at an intersection between anapproximate straight line of a flat region of log G′ and an approximatestraight line of a region where log G′ sharply decreases was taken asthe glass transition temperature (G′Tg).

[90-Degree Bending Strength]

The formed plate obtained in each of the examples and comparativeexamples was cut into a size of 63 mm in length (in the directionorthogonal to the fiber)×12.7 mm in width (in the direction parallel tothe fiber) by a wet diamond cutter, and the end face was polished with#1000 sandpaper to produce a test piece. The test piece was subjected toa three-point bending test using a universal tester (manufactured byInstron, Instron 4465, analysis software: Bluehill) according to ASTMD790 under the conditions of an indenter R: 5.0, L/D: 16, and acrosshead speed: 0.92 to 0.94 mm/min, and the 90-degree bending strengthwas calculated.

[Raw Materials Used]

Raw materials used are shown below.

(Epoxy Resin)

Epoxy resins a-1 and a-2 having the structure represented by Formula (1)in a molecule

a-1: Reaction Product of an Epoxy Resin and 4,4′-Diaminodiphenylsulfone

A reaction product obtained by mixing a bisphenol A type epoxy resin(product name “jER 828”, manufactured by Mitsubishi ChemicalCorporation) and 4,4′-diaminodiphenylsulfone (product name: SEIKACURE-S,manufactured by Wakayama Seika Kogyo Co., Ltd.) in a mass ratio of 100:9at room temperature and thereafter mixing and heating the mixture at150° C., which is a mixture primarily containing a reaction product ofan epoxy resin and an amine compound having at least one sulfur atom ina molecule (epoxy equivalent weight 266 g/eq, viscosity (90° C.) 1.3Pa·s).

a-2: Dissolved Product of a-1 and Polyethersulfone

A resin composition (I) was obtained by mixing jER 828/polyethersulfone(product name: E2020P, manufactured by BASF)=7/3 (unit: parts by mass)and uniformly dissolving the mixture at 180° C. By mixing 71.67 parts bymass of the a-1 and 23.33 parts by mass of the resin composition (I), adissolved product of the a-1 and polyethersulfone was obtained. Epoxyequivalent weight 266 g/eq, viscosity (90° C.) 1.3 Pa·s)

N775: Phenol novolac type epoxy resin (product name “N-775”, epoxyequivalent weight 189, manufactured by DIC Corporation, proportion ofmolecules having three or more epoxy groups 90.9%).

jER 828: Bisphenol A type epoxy resin (product name “jER 828”, epoxyequivalent weight 189, manufactured by Mitsubishi Chemical Corporation).

jER 1032H60: Trisphenolmethane type epoxy resin (product name “jER1032H60”, epoxy equivalent weight 169, manufactured by MitsubishiChemical Corporation, proportion of molecules having three or more epoxygroups 98.4%).

jER 604: Bisphenol A type epoxy resin (product name “jER 604”, epoxyequivalent weight 120, manufactured by Mitsubishi Chemical Corporation,proportion of molecules having three or more epoxy groups 100%).

(Epoxy Resin Curing Agent)

Dicy 15: Dicyandiamide (product name “Dicy 15”, manufactured byMitsubishi Chemical Corporation, average particle size 8.3 μm)

Dicy 1400F: Dicyandiamide (product name “Dicyanex 1400F”, manufacturedby Air Products, average particle size 4.5 μm)

(Imidazole Compound)

2MAOK:2,4-diamino-6-(2′-methylimidazolyl-(1′))-ethyl-s-triazine/isocyanuricacid addition product (volume average particle size 1.1 μm, product name“CUREZOL 2MAOK-PW”), manufactured by Shikoku Chemicals Corporation).

2MZA: 2,4-diamino-6-(2′-methylimidazolyl-(1))-ethyl-s-triazine (volumeaverage particle size 1.4 μm, product name “CUREZOL 2MZA-PW”,manufactured by Shikoku Chemicals Corporation).

(Epoxy Resin Curing Accelerator)

TBDMU: 2,4-bis(3,3-dimethylureido)toluene (product name “Omicure 24”,manufactured by PTI Japan, Ltd.).

DCMU: 3-(3,4-dichlorophenyl)-1,1-dimethylurea (product name “DCMU 99”,manufactured by Hodogaya Chemical Co., Ltd.).

(Reinforcing Fiber)

Carbon fiber bundle: product name “TRW40 50L”, manufactured byMitsubishi Chemical Corporation, tensile strength 4.1 GPa, tensilemodulus 240 GPa, number of filaments 50,000, basis weight 3.75 g/m.

(Curing Start Temperature)

The curing start temperature was calculated as follows.

The curing start temperature was set to a temperature at an intersectionbetween a tangent at an inflection point and a baseline in a DSC curveobtained by measuring a calorific value by a differential scanningcalorimeter (DSC) at a temperature rising rate of 10° C./min of a sampleresin composition prepared by mixing 10 parts by mass of the targetimidazole compound with 100 parts by mass of a bisphenol A type epoxyresin having an epoxy equivalent weight of 189 (jER 828 manufactured byMitsubishi Chemical Corporation).

The curing start temperature of 2MAOK was 149.1° C., and the curingstart temperature of 2MZA was 139.1° C.

(Preparation of Masterbatch)

The components were kneaded at the mass ratios shown in Table 1 below,and then uniformly dispersed with three rolls to prepare masterbatchesI-1 to I-7 and II-1 to II-4.

TABLE 1 Masterbatch I-1 I-2 I-3 I-4 I-5 I-6 I-7 II-1 II-2 II-3 II-4Epoxy resin jER828 12 12 12 12 12 12 12 12 12 12 12 Epoxy resin curingagent Dicy15 5 Dicy1400F 5 5 5 5 2 5 5 5 5 Epoxy resin curingaccelerator TBDMU 2 2 2 3 2 2 8 4 4 DCMU 2 Imidazole compound 2MAOK 3 32 2 8 2 4 3 2 2MZA 2

Example 1

60 parts by mass of the epoxy resin a-2, 25 parts by mass of N775, and10 parts by mass of jER 828 were put into a dissolving pot and dissolvedat 80° C. Thereafter, the resultant was cooled to 60° C., 22 parts bymass of the masterbatch I-1 was added thereto, and the resultant wasfurther stirred and mixed at 60° C., thereby obtaining a thermosettingresin composition (C-1).

Table 2 shows the evaluation results of the obtained thermosetting resin(C-1).

The thermosetting resin composition (C-1) was applied onto release paperat 60° C. using Multicoater (M-500 type, manufactured by HIRANO TECSEEDCo., Ltd.), thereby obtaining a resin film. A carbon fiber bundle waswound on the resin film on a drum winder and then sandwiched between theresin film and the same film to cause the carbon fiber bundle to beimpregnated with the thermosetting resin composition (C-1), therebyobtaining a unidirectional prepreg. The basis weight of the carbon fiberbundle was 244 g/m² and the resin content was 31.0 mass % in theprepreg.

Ten prepregs obtained were laminated with their fiber directionsaligned, and were press-formed (forming temperature: 140° C., formingtime: 5 minutes), thereby obtaining a formed plate (fiber-reinforcedcomposite material).

Table 2 shows the evaluation results of the obtained formed plate.

Examples 2 to 7

Thermosetting resin compositions (C-2) to (C-7) were prepared in thesame manner as in Example 1, except that the component composition andthe resin content were changed as shown in Table 2.

Using the obtained thermosetting resin compositions (C-2) to (C-7),resin films were prepared in the same manner as in Example 1, therebyobtaining formed plates.

Table 2 shows the evaluation results of the obtained thermosetting resincompositions (C-2) to (C-7), and the formed plates obtained from thethermosetting resin compositions.

Comparative Examples 1 to 4

Thermosetting resin compositions (X-1) to (X-4) were prepared in thesame manner as in Example 1, except that the component composition andthe resin content were changed as shown in Table 2.

Using the obtained thermosetting resin compositions (X-1) to (X-4),resin films were prepared in the same manner as in Example 1, therebyobtaining formed plates.

Table 2 shows the evaluation results of the obtained thermosetting resincompositions (X-1) to (X-4) and the formed plates obtained from thethermosetting resin compositions.

TABLE 2 Example Comparative Example 1 2 3 4 5 6 7 1 2 3 4 Thermosettingresin composition C-1 C-2 C-3 C-4 C-5 C-6 C-7 X-1 X-2 X-3 X-4 Epoxyresin a-1 45 a-2 60 55 65 65 60 60 55 55 55 55 N775 25 30 10 30 25 25 3030 30 30 jER828 10 10 20 10 10 10 10 10 10 jER1032H60 30 jER604 20Masterbatch 22 22 21 22 21 24 27 20 21 20 21 (I-1) (I-2) (I-3) (I-4)(I-5) (I-6) (I-7) (II-1) (II-2) (II-3) (II-4) Content (%) of a-1 or a-2in total epoxy resin 44.1 40.3 48.0 42.1 48.0 44.1 44.1 40.3 40.3 40.340.3 Content (%) of polyfunctional epoxy resin 22.2 26.5 28.5 27.6 26.722.2 22.2 26.5 26.5 26.5 26.5 Isothermal viscosity stability V0/V1 1.901.35 1.65 2.37 3.22 2.62 3.04 4.88 1.59 1.08 1.33 Evaluation of 60° C.isothermal AA AA AA A B B B C AA AA AA viscosity stability Curingcompletion time (min) 4.38 4.52 5.46 5.40 5.23 4.77 4.07 7.01 6.57 6.556.85 Glass transition temperature (° C.) 151 154 163 158 163 — — 142 135160 148 90-degree bending strength 101 110 118 110 118 — — 96 103 47 53

In Table 2, the numerical values of the raw materials indicate parts bymass used. The symbol in parentheses in the masterbatch indicates thetype of the masterbatch used, and the numerical value indicates parts bymass of the masterbatch used.

As shown in Table 2, in Examples 1 to 5 using the thermosetting resincompositions (C-1) to (C-5), it was shown that the fiber-reinforcedcomposite materials were resin compositions that had both rapidcurability that enabled high cycle press forming and storage stabilityrequired for the production of prepreg while achieving both high heatresistance and high 90-degree bending strength.

In Examples 6 and 7, it was shown that rapid curability that enabledhigh cycle press forming was exhibited.

In Comparative Example 1 using the thermosetting resin composition(X-1), since dicyandiamide or a derivative thereof was not contained,stability at 60° C. was low, and rapid curability suitable for highcycle press forming was not provided. Furthermore, the obtained formedplate was low in both heat resistance and 90-degree bending strength.

In Comparative Example 2 using the thermosetting resin composition(X-2), since an imidazole compound was not contained, rapid curabilitysuitable for high cycle press forming was not provided. Furthermore, theobtained formed plate had low heat resistance.

In Comparative Examples 3 and 4 using the curable resin composition(X-3) or the curable resin composition (X-4), since a urea derivativehaving two dimethylureido groups in a molecule was not contained, rapidcurability suitable for high cycle press forming was not provided.Furthermore, the obtained formed plate had low 90-degree bendingstrength.

INDUSTRIAL APPLICABILITY

The present invention is applicable to various uses, and is particularlyuseful as a material for industrial use, especially for vehicles.

REFERENCE SIGNS LIST

-   -   1: Prepreg laminate    -   2: Fiber-reinforced composite material    -   3: Flat plate portion    -   4: Side portion    -   10: Prepreg    -   100: Die    -   110: Lower die    -   112: Protruding portion    -   120: Upper die    -   122: Recessed portion

The invention claimed is:
 1. A thermosetting resin compositioncomprising: an epoxy resin; an epoxy resin curing agent; an imidazolecompound; and an epoxy resin curing accelerator, wherein the epoxy resincuring agent comprises dicyandiamide or a derivative thereof, the epoxyresin curing accelerator comprises a urea derivative having two or moredimethylureido groups in a molecule, the thermosetting resin compositioncomprises 1 to 9 parts by mass of the dicyandiamide or a derivativethereof, 1 to 8 parts by mass of the imidazole compound, and 1 to 8parts by mass of the urea derivative having two or more dimethylureidogroups in a molecule, each with respect to 100 parts by mass of theepoxy resin, and the epoxy resin comprises 30 to 70 parts by mass of afirst epoxy resin having a structure represented by Formula (1) in amolecule, and 20 to 40 parts by mass of a second epoxy resin, eachrelative to 100 parts by mass of the epoxy resin,

wherein the second epoxy resin is a polyfunctional epoxy resin havingthree or more epoxy groups in a molecule, and the polyfunctional epoxyresin does not contain the structure represented by Formula (1).
 2. Thethermosetting resin composition according to claim 1, wherein a curingcompletion time is 6 minutes or shorter at 140° C.
 3. The thermosettingresin composition according to claim 1, wherein, when a viscosity afterholding at 60° C. for 5 minutes is V0 and a viscosity after holding at60° C. for 5 hours is V1, V1/V0 is 1.0 to 4.5.
 4. The thermosettingresin composition according to claim 1, wherein the imidazole compoundcomprises an imidazole compound having a curing start temperature of100° C. or higher.
 5. The thermosetting resin composition according toclaim 1, wherein the imidazole compound comprises at least one selectedfrom the group consisting of an imidazole derivative, an imidazoleadduct, an inclusion compound of imidazole, a microencapsulatedimidazole, and an addition product comprising imidazole.
 6. Thethermosetting resin composition according to claim 1, wherein the ureaderivative having two or more dimethylureido groups in a moleculecomprises one or both of 2,4-bis(3,3-dimethylureido)toluene and4,4′-methylene bis(phenyldimethylurea).
 7. The thermosetting resincomposition according to claim 1, wherein the polyfunctional epoxy resinhaving three or more epoxy groups in a molecule comprises at least oneselected from the group consisting of a phenol novolac type epoxy resin,a trisphenolmethane type epoxy resin, and a tetraglycidyldiaminodiphenylmethane.
 8. A prepreg comprising: the thermosetting resincomposition according to claim 1; and a reinforcing fiber base material.9. The prepreg according to claim 8, wherein a cured product obtained byholding the prepreg at 140° C. for 5 minutes while applying a pressurethereto has a glass transition temperature of 150° C. or higher.
 10. Afiber-reinforced composite which is a cured product of the prepregaccording to claim
 8. 11. The fiber-reinforced composite according toclaim 10, which has a glass transition temperature of 150° C. or higher.12. A method of producing a fiber-reinforced composite, the methodcomprising: curing a prepreg according to claim 8 to produce thefiber-reinforced composite.
 13. The method according to claim 12,wherein the fiber-reinforced composite has a glass transitiontemperature of 150° C. or higher.
 14. A method of producing afiber-reinforced composite, the method comprising: subjecting theprepreg according to claim 8 to heating and pressurization forming. 15.The method according to claim 14, the method further comprising: shapingthe prepreg according to claim 8, prior to the heating and pressurizingforming, to produce a preform having a near net shape of the fiberreinforced composite which is to be produced by the heating andpressurizing forming.
 16. The method according to claim 14, wherein thefiber-reinforced composite has a glass transition temperature of 150° C.or higher.